The file RefGenomeOrthologs contains orthologs, and within-species paralogs, for
each of the 12 Reference Genomes of the Gene Ontology Consortium: human, mouse, rat,
chicken, zebrafish, C. elegans, Drosophila, yeast, fission yeast, Dictyostelium,
Arabidopsis, E. coli.  The orthologs are determined based on the PANTHER
phylogenetic trees.

Orthology is defined between pairs of genes per Fitch, 1971.  This file lists pairs
of genes with relationships as defined below.

First and second column: identifiers, pipe-delimited, of the gene pair.  First is
the UniProt 5-letter species code, second is the gene identifier, third is the
protein sequence identifier.

Third column: relationship type.  LDO (least-diverged ortholog), O (ortholog), P
(within-species paralog).  LDO i